The value of methane as a potential fuel source has long been recognized and exploited. Because the current price of natural gas is about the same as petroleum in terms of BTUs, however, natural gas has evolved from a low-cost fuel source that was often a by-product of petroleum oil field production to a fuel source worth drilling for. The increasing value of natural gas has been driven by a number of factors, including the world's shrinking supply of petroleum reserves and the increasingly stricter environmental regulations placed on coal fired power plants. Generating electricity from coal, for example, releases twice the carbon dioxide as making it from natural gas. Burning coal also produces mercury vapor that has been estimated to contribute to over twenty percent of the world's mercury pollution. In addition, the burning of coal can release arsenic compounds and sulfur dioxide. As a result, expensive pollution control systems are now required for all new coal fired plants and most existing plants.
The demand for gases that contain methane, such as natural gas and synthetic natural gas, will likely continue to increase in the future, not only because of the increasing demand for cleaner burning fuels, but also because the world's demand for petroleum will continue to drive its price higher, particularly as known petroleum reserves are depleted. The demand for methane will also likely increase as hydrogen fuel cells are commercialized. This is because the least expensive process for producing hydrogen involves chemically converting methane and water to hydrogen and carbon dioxide in the presence of a catalyst.
To satisfy demand for natural gas, the U.S. currently imports about 2 billion cubic feet of liquid natural gas (“LNG”) per day. Importing LNG has at least two significant draw backs. First, the cost of importing LNG is high. Second, it is risky to have large LNG terminals located in major seaports in the present world environment, as such terminals could be the subject of terrorist attack. Even in the absence of terrorism, however, such terminals pose a significant explosion risk.
Because of the world's increasing demand for methane gas and cleaner burning fuels in general, alternative sources of gaseous fuels that contain methane are needed. As a result, an economical technique for producing gaseous fuels that contain methane and/or a liquid synfuel, such as synthetic petroleum, would have significant market value.
Artificial gas for use as heating fuel and derived from coal or coke was widely used during the latter part of the nineteenth century and during the first few decades of the twentieth century in the U.S. Because of the great availability and, at one time, apparent inexhaustible supply of natural gas in the U.S., as well as a few other areas in the world, manufactured gases were phased out rapidly. The use of natural gas, on the other hand, increased 730% between 1940 and 1970 in the U.S. During this period, the U.S. gas industry produced 313 trillion cubic feet of natural gas. However, in other parts of the world where natural gas was in short supply locally, manufactured gas has persisted.
Historically, the gasification of coal has involved the heating of coal through pyrolysis, carbonization, or retorting to cause its decomposition and gasification. The gas resulting from the gasification process typically contains varying concentrations of carbon monoxide, carbon dioxide, methane, and hydrogen, the concentration of each constituent depending on the particular gasification technique employed. Thus, while gasification is the key step in these heat based gasification techniques, it must be appreciated that it is only one step of the overall process of forming a manufactured or synthetic natural gas (“SNG”) from coal. In addition to the gasification step, such processes typically include gas conditioning, gas purification, methanation and by-product treatment processes. Further, purification of the product gas from the coal gasification step to a degree of purity required for methane synthesis is difficult due to the large quantities and variety of impurities in the gas.
Another draw back of coal gasification using known heat gasification techniques is that the gasification process is a highly endothermic process, and the heat requirements of the process have to be covered by the addition of heat. This may be accomplished through, for example, direct heat supplied through the partial combustion of the coal with oxygen or indirect heat supplied from an external fuel source. In either event, however, a significant portion of the overall energy values contained in the coal prior to gasification are expended to gasify the coal. Finally, the heat based gasification processes do not appear to have any applicability to animal or plant waste, both significant renewable resources of carbonaceous materials.
The biological conversion of organic matter into methane has also been studied for many years. The degradation of organic matter to methane and carbon dioxide (i.e., methanogenic degradation) occurs in limited oxygen or other electron acceptor environments. This process is widespread in swamps, rice paddies, peat bogs, and in the intestinal tract of ruminant animals and plays a major role in the global carbon cycle. Indeed, the total biological methanogenic production of methane is estimated at 500 million tonnes per year, making methane the second most abundant greenhouse gas.
Methanogenic degradation is slower and less exergonic than aerobic degradation. However, aerobic degradation does not produce methane. More importantly, methanogenic conversion only releases about 15% of the energy that is released by complete aerobic conversion of the same organic carbon compound to carbon dioxide and water. This is because the remaining 85% of the energy is stored in the resulting methane for subsequent oxidation.
Methanogens are a group of Archaea that produce methane in anaerobic or anoxic environments. They are obligate anaerobes, and thus cannot tolerate any molecular or ionic oxygen in their environment. They form an interdependent relationship with other organisms, including bacteria, protozoa, insects, and grass feeding animals, such as cows. They use simple organic compounds (e.g., formate, acetate, methyl-amines and several alcohols) produced by those organisms and carbon dioxide as an energy source to produce methane.
Although methanogens depend on fermentative organisms to produce the simple organic substrates on which they rely for energy, fermentative microorganisms likewise depend on methanogens to remove hydrogen and the simple organic compounds they produce to improve their energetics. This interdependence is called syntrophic cooperation. In this cooperative relationship, the fermentative microorganism species ferment long chain organic carbon molecules to H2 and C-1 and C-2 compounds for the methanogens to feed upon. This fermentation process is inhibited by the H2 and C-1 and C-2 compounds produced. Methanogens, however, oblige these fermenting species by removing the hydrogen and C-1 and C-2 compounds as they convert them to methane. As a result, the syntrophically cooperating anaerobes cooperate in the conversion of complex organic matter to methane and carbon dioxide with very little loss of the energy values contained in the original organic matter. Recent advances in molecular biology have led to a better understanding of this complex, but widespread natural process.
A more in depth review of methanogenic degradation and a list of some methanogenic microorganisms is provided in B. Schink, Energetics of Syntrophic Cooperation in Methanogenic Degradation, Microbiology and Molecular Biology Reviews, 61:262-280 (June 1997), which is hereby incorporated by reference.
Long before the biology of methanogenic degradation was understood, people attempted to exploit methanogenic degradation to produce methane for its fuel value. For example, U.S. Pat. No. 1,990,523, which issued to Buswell et al. in 1935, describes a method for methane generation using anaerobic bacteria conversion of sewage.
Much effort has also been devoted to developing in situ microbial processes for the conversion of low-grade fossil fuels to methane. For example, U.S. Pat. No. 3,826,308, which issued to Compere-Whitney in 1974, and U.S. Pat. No. 5,424,195, which issued to Volkwein in 1995, focused on treating very low-grade coal that was left behind in underground mines. In U.S. Pat. No. 6,543,535, which issued to Converse et al. in 2003, a process for the in situ bioconversion of hydrocarbons to methane in hydrocarbon-bearing formations is described. The process described in the Converse et al. patent includes altering the environment of the hydrocarbon bearing formations so as to stimulate the growth of native microbes found within the formations.
Some of the subterranean microbial hydrocarbon conversion processes described in the literature have also used explosives in an effort to increase the surface area of coal or oil shale deposits being microbially treated. The explosions create what is called a “rubble chimney.” While formation of a rubble chimney increases the rate of conversion to methane, the overall conversion rate remains relatively slow.
Biological processes have also been used to aid in the recovery of petroleum from oil reserves. For example, U.S. Pat. No. 2,413,278, which issued to Zobell in 1946, U.S. Pat. No. 2,807,570, which issued to Updegraff in 1957, and U.S. Pat. No. 2,907,389, which issued to Hitzman in 1959, teach ways to use bacteria to generate extra recovery of petroleum from oil reservoirs after 40 to 50% of the contained oil has been removed by pumping and water flooding. The process of using bacteria to recover additional oil from underground reservoirs is called Microbial Enhanced Oil Recovery (MEOR).
One of the major limitations of in situ microbial gasification and MEOR processes is not the general ability of the bacteria used in those processes to dislodge oil, reduce viscosity or convert oil to methane, but rather the problems encountered with providing the right environment for microbial growth in the deep underground reservoirs or formations. Within such environments a variety of environmental factors may be encountered that individually or collectively inhibit to varying degrees, or even prevent, the microbial conversion or degradation process. Such environmental challenges can include, for example, high temperatures, high concentrations of salts or other biocides, and limited porosity of the native rock in which the oil is being held, as this will restrict the accessibility of microbes to the oil. And though it may be possible to modify the environment of a formation to some degree, sometimes it will not be possible or practical to alter the environment of a formation sufficiently to have a practical impact on microbial activity.
A need exists, therefore, for an ex situ process that is capable of converting vast quantities of low-grade fossil fuels, as well as other organic carbonaceous materials, into one or more synfuels, including methane and/or oil. While biodegradation of low-grade fossil fuels can theoretically be performed in stirred tank bioreactors, due to the relatively long residence time that will be necessary to convert such carbonaceous materials to oil and/or a gaseous fuel and the large amount of material that will need to be processed to yield relatively small quantities of fuel values, the cost of scaling a stirred tank processes up to commercial scale is simply too high to make stirred tank bioreactors a practical option. On the other hand, a very large, low-cost, yet relatively efficient, heap bioreactor could economically unlock the trillions of barrels of oil in the world's resources of oil shale and oil sands. Such a bioreactor could also be used in the biogasification of other organic carbonaceous materials, including renewable resources such as plant and animal wastes, as well as other non-renewable resources such as coal. The synfuel (e.g., methane, alcohol, and/or synthetic petroleum oil) produced in such bioreactors could help fuel an energy-hungry world for the rest of the century.
In view of the foregoing, one object of the present invention is to provide a new bioreactor design for use in converting organic carbonaceous materials into synfuel. Another, and separate, object is to provide a new method for converting organic carbonaceous materials into synfuel.